This invention relates to a process for producing a catalyst useful in the hydrotreatment of hydrocarbon oils and to a catalyst produced by such a process as well as to the use of such a catalyst in the hydrotreatment of hydrocarbon oils. More particularly, this invention relates to a process for producing a hydrotreating catalyst comprising an alumina or an alumina-containing carrier which also contains a specified silica content and preferredly a specified pore size distribution, this carrier supporting hydrogenation-active components in a well-dispersed state.
In the description of this invention, the terms "hydrotreating" or "hydrotreatment" refer to various processes for treating a hydrocarbon oil by contact with hydrogen, which include hydrofining under reaction conditions of relatively low severity, hydrofining under reaction conditions of relatively high severity accompanied with an appreciable cracking reaction, hydroisomerization, hydrodealkylation, and other reactions of hydrocarbon oils in the presence of hydrogen. Hydrodesulfurization, hydrodenitrogenation, and hydrocracking of distillates and residual oils from atmospheric and vacuum distillation processes are examples of such processes as are the hydrofining of a kerosene fraction, a gas oil fraction, wax, or lubricating oil fractions.
Since the catalyst produced by this invention is suitably used in performing hydrodesulfurization of atmospheric middle distillates such as kerosene fraction and gas oil fraction, vacuum heavy distillates, residual oils containing asphalt, or mixtures thereof, the explicit description in this specification of the use of the catalyst will be made chiefly in connection to hydrodesulfurization.
Desulfurization processes comprising hydrofining a sulfur-containing hydrocarbon oil in the presence of a catalyst containing hydrogenation-active components have long been known, and nowadays the establishment and improvement of a hydrodesulfurization process whereby heavy hydrocarbon oils containing asphalt and metal-containing compounds can be desulfurized industrially is eagerly desired as a nonpolluting process. Moreover, the residual oils contain nitrogen compounds in addition to the sulfur compounds and as a result nitrogen oxides are formed upon combustion of such oils. The oxides of both sulfur and nitrogen are undesirable atmospheric pollutants.
The sulfur and nitrogen compounds contained in hydrocarbon oils can be removed in the forms of hydrogen sulfide and ammonia, respectively, by a hydrogenation reaction in the presence of a catalyst.
However, heavy hydrocarbon oils, especially residual oils, contain catalyst poisoning substances, for example, organic compounds of metals such as vanadium or iron, which remarkably lower the activity of a catalyst and further prevent its regeneration. Further, residual oils contain high-molecular compounds of polynuclear aromatics such as asphaltene and asphalt, and these compounds cover the active sites of the catalyst, and cause the formation of carbonaceous deposits which block the pores of the catalyst so as to lower its activity. Thus, the hydrofining of a heavy hydrocarbon oil must cope with extremely difficult technical problems.
Heretofore, a variety of investigations have been made about catalysts for use in the hydrodesulfurization of residual oils. For example, a process is known in which a catalyst having a pore size distribution in which the volume of pores having a diameter of at least 80 Angstroms is limited to below 10 percent of the total pore volume is used to prevent intrusion of the asphalt and metal-containing compounds contained in the feed oil into the pores. Similarly, a process is known in which a catalyst in which pores having a diameter of below 120 Angstroms are distributed relatively uniformly at intervals of 10 Angstroms. Further, there is also disclosed a catalyst for hydrodesulfurizing crude petroleum or topped crude petroleum in which the volume of pores having a diameter in the range of about 50 to 100 Angstroms accounts for at least 50 percent of the total pore volume and the volume of pores having a diameter in the range of 0 to 50 Angstroms accounts for at most 25 percent of the total pore volume.
Additionally, in the hydrodesulfurization in the petroleum refining, there has heretofore been widely used a hydrotreating catalyst produced by allowing an alumina carrier to support at least one metal selected from the group consisting of Group VIII metals of the Periodic Table of Elements and/or Group VIB metals of the Periodic Table of Elements, for example, a cobalt/molybdenum catalyst or a nickel/molybdenum catalyst. It is the most important factor in the production of such a hydrotreating catalyst to allow a carrier to support a large amount (about 20 percent or more by weight in terms of an oxide) of molybdenum/cobalt or molybdenum/nickel particles of a uniform composition in a highly dispersed state.
Proposed processes for producing a hydrotreating catalyst include (1) a coprecipitation process, (2) a kneading process, and (3) an impregnation process, all of which have been generally put into practice. The coprecipitation process, however, has a drawback that when a plurality of active metal components are used as in the case of the above-mentioned hydrotreating catalyst, it is difficult to disperse these active metal components uniformly and to produce a catalyst with good reproducibility. The kneading process also has a drawback that although it is necessary to ensure sufficient access of a plurality of active metal components to a carrier as well as their uniform and perfect mixing in producing a hydrotreating catalyst having a plurality of active metal components as above, attainment of such a state is extremely difficult in this process. As compared with the above two processes, the impregnation process may be thought suitable for the production of a hydrotreating catalyst having a plurality of active metal components as described above, for example, an alumina-supported cobalt/molybdenum or nickel/molybdenum catalyst.
In the production of a hydrotreating catalyst by the impregnation process, a process (two-step impregnation process) has been proposed which comprises using alumina generally as a carrier, allowing the carrier to support first molybdenum (first step), and then allowing the carrier to support cobalt or nickel in the form of fine particles of molybdenum/cobalt or molybdenum/nickel which are formed by the interaction between said active metal and molybdenum (second step). However, it is difficult to allow the carrier to support molybdenum in an amount of as large as about 15 percent by weight in a highly dispersed state in the first step, and the dispersibility in the catalyst obtained finally is virtually determined by the state of dispersion of molybdenum in said first step, which inevitably leads to decrease in activity.
The above mentioned problem of the impregnation process may be solved by using a process which comprises allowing an alumina carrier to support cobalt or nickel in the first step, and then allowing the carrier to support molybdenum, but no desired performances can be obtained in this case because when active metal components such as cobalt or nickel are supported on the alumina carrier, they do not show sufficient interaction with the alumina carrier and lose their activity by forming agglomerates of inactive Co.sub.3 O.sub.4 or cobalt aluminate.
Further, a process comprising allowing an alumina carrier to support a plurality of active metals simultaneously by impregnation is not preferred because it is difficult to distribute all the components uniformly throughout the carrier.
As described above, it is extremely difficult in producing a catalyst containing large amounts of supported active metal components and carrying a plurality of active metal components, such as a hydrotreating catalyst, to allow the carrier to support all the components in a uniform and highly dispersed state by the conventional methods.